Substituted starches are starch derivatives that have been chemically reacted to replace one or more of the hydroxyl functional groups. Typically, the process involves etherification or esterification of a starch or modified starch which append ether or ester linkages along the starch polymer backbone. This process differs from more traditional modifications made to starches such as oxidization, acid-thinning, cross-linking, and pre-gelatinization. The starch may come from one of many natural sources, such as potato, tapioca, or corn. In fact, any of numerous starches are well known and commercially available in a variety of forms, including liquids, particles and fine powders. A substituted starch may also be modified in another way, such as acid-thinning, prior to or after substitution with one or more types of functionalities. For example, substituent groups may be alkyl as in methyl or ethyl substitution, hydroxyalkyl as in hydroxyethyl, hydroxymethyl, or hydroxypropyl substitution, hydrophobic, cationic, anionic, or combinations of these. Regardless, methods of preparing substituted starches is well known.
Acid-thinned or pre-gelatinized starches are sometimes added to wallboard core formulations to improve the bonding of the wallboard core to the paper facing. These starches typically migrate to the surface or are applied directly to the surface of the wallboard core. In conventional wallboard, the paper facing resists most of the stress, and a reliable bond between the paper facing and the wallboard core is essential to obtain strength and durability of a wallboard. Also, it is well known and accepted that such starches do little or nothing to significantly strengthen the wallboard core.
Indeed, most starches either do not dissolve and disperse in the inorganic matrix or migrate efficiently to the surfaces during drying. Thus, such starches serve no known role in strengthening the wallboard core. Even starches that remain predominantly dispersed throughout the composite do not adequately bind the discrete inorganic phase, which may be of any morphology, including needle-like crystals, particulates, or fibers. Many attempts have been made to find an inexpensive and useful additive for strengthening the wallboard core, but such attempts have failed to provide properties that are substantially better than conventional wallboard.